Resistive sensors are used in many applications, such as thermal detection and thermal imaging systems. A thermal imaging system or camera uses a large array of sensors to construct an image, wherein the new images are generated at short intervals to create a moving picture. Due to physical variations between the sensors, each pixel must be calibrated individually before the pixel outputs are combined into a picture. In other thermal sensor systems calibration may also be necessary to avoid saturating the read-out electronics. Thermal sensors are well known to have high temperature sensitivity, and thus, are significantly affected by changes in ambient temperature as well as changes in the incident thermal radiation that is to be measured. Accordingly, a shielded or thermally shorted reference sensor may be used to mitigate the sensitivity to operating conditions by skimming off such variations caused by environmental changes.
FIG. 1 shows the prior art technique of having a current skimming sensor bridge with digital calibration electronics. The active sensor Ract 101 is subjected to incident thermal radiation which affects and alters its resistance. To reduce the sensitivity to operating conditions, the current is skimmed off through a reference resistor or sensor element Rref 102, with the same nominal resistance and similar thermal properties to Ract 101. In a thermal imaging array, Rref 102 can be shielded or thermally shorted. The output current is given by:
                                                                        I                out                            =                            ⁢                                                                    V                                          bias                      ⁢                                                                                          ⁢                      1                                                                            R                    act                                                  -                                                      V                                          bias                      ⁢                                                                                          ⁢                      2                                                                            R                    ref                                                                                                                          =                            ⁢                                                I                  act                                -                                                      I                    ref                                    .                                                                                        (        1        )            The output current is converted to a voltage by a current-to-voltage (IV) amplifier 105 and converted to digital data with an analog-to-digital converter (ADC) 106 before being fed to the digital output 107.
Since Vbias1 and Vbias2 are controlled by Vb1 and Vb2 through the bias transistors M1 103 and M2 104, the calibration processor 109 can calibrate for mismatches between the active sensor and the reference element by adjusting Vb1 and Vb2 individually, using digital algorithms based on an analysis of the ADC output. However, the calibration processor is a digital signal processor (DSP) that produces digital data. The data must be converted to voltages Vb1 and Vb2 by means of digital-to-analog converters (DACs) 108a and 108b. Therefore, the resolution of the mismatch calibration is limited by the resolution of the DACs used to control the biasing, and consequently, the required resolution of the mismatch calibration must be at least as high as that of the signal chain and ADC. In applications involving large sensor arrays, such as thermal imaging systems, the implementation is substantially complex because the typical sensors are simultaneously biased and a large number of high resolution DACs will use a lot of power and chip area.
Based on the foregoing, there is a need for resistive sensor bridges which may be used with thermal detection and thermal imaging systems and overcomes deficiencies of the prior art. More specifically, there is a need for a resistive sensor bridge with calibration and compensation schemes which are less complex and provide better resolution.